General I/O Parameters

maxphys

Description

Defines the maximum size of physical I/O requests. If a driver encounters a request larger than this size, the driver breaks the request into maxphys sized chunks. File systems can and do impose their own limit.

Data Type

Signed integer

Default

131,072 (sun4u or sun4v) or 57,344 (x86). The sd driver uses the value of 1,048,576 if the drive supports wide transfers. The ssd driver uses 1,048,576 by default.

Range

Machine-specific page size to MAXINT

Units

Bytes

Dynamic?

Yes, but many file systems load this value into a per-mount point data structure when the file system is mounted. A number of drivers load the value at the time a device is attached to a driver-specific data structure.

Validation

None

When to Change

When doing I/O to and from raw devices in large chunks. Note that a DBMS doing OLTP operations issues large numbers of small I/Os. Changing maxphys does not result in any performance improvement in that case.

You might also consider changing this parameter when doing I/O to and from a UFS file system where large amounts of data (greater than 64 KB) are being read or written at any one time. The file system should be optimized to increase contiguity. For example, increase the size of the cylinder groups and decrease the number of inodes per cylinder group. UFS imposes an internal limit of 1 MB on the maximum I/O size it transfers.

rlim_fd_max

Specifies the “hard” limit on file descriptors that a single process might have open. Overriding this limit requires superuser privilege.

Data Type

Signed integer

Default

65,536

Range

1 to MAXINT

Units

File descriptors

Dynamic?

No

Validation

None

When to Change

When the maximum number of open files for a process is not enough. Other limitations in system facilities can mean that a larger number of file descriptors is not as useful as it might be. For example:

A 32-bit program using standard I/O is limited to 256 file descriptors. A 64-bit program using standard I/O can use up to 2 billion descriptors. Specifically, standard I/O refers to the stdio(3C) functions in libc(3LIB).

select is by default limited to 1024 descriptors per fd_set. For more information, see select(3C). A 32-bit application code can be recompiled with a larger fd_set size (less than or equal to 65,536). A 64-bit application uses an fd_set size of 65,536, which cannot be changed.

An alternative to changing this on a system wide basis is to use the plimit(1) command. If a parent process has its limits changed by plimit, all children inherit the increased limit. This alternative is useful for daemons such as inetd.

Commitment Level

Unstable

rlim_fd_cur

Description

Defines the “soft” limit on file descriptors that a single process can have open. A process might adjust its file descriptor limit to any value up to the “hard” limit defined by rlim_fd_max by using the setrlimit() call or by issuing the limit command in whatever shell it is running. You do not require superuser privilege to adjust the limit to any value less than or equal to the hard limit.

Data Type

Signed integer

Default

256

Range

1 to MAXINT

Units

File descriptors

Dynamic?

No

Validation

Compared to rlim_fd_max. If rlim_fd_cur is greater than rlim_fd_max, rlim_fd_cur is reset to rlim_fd_max.

When to Change

When the default number of open files for a process is not enough. Increasing this value means only that it might not be necessary for a program to use setrlimit to increase the maximum number of file descriptors available to it.